This list of aquatic alien species is limited to species and groups of genetically related extraterrestrial organisms that rely partially or exclusively on inhabiting an aquatic environment, while excluding specific references to individual organisms. This list includes sapient, non-sapient and hybrid forms of aquatic alien species.
Antedeans from Antede III are an ichthyohumanoid species which resemble fish. While a member of the United Federation of Planets, they seldom receive visitors and thus have not been seen by many members of the Federation. Another trait that keeps them from interacting with their fellow Federation members is a strong distaste for space flight: although the Antedeans are a space-going race, space travel is quite traumatic for them. In order for individuals to deal with this problem they induce a catatonic state while in space flight. To awaken from this state takes several hours. Once awakened from the sleep Antedeans are generally ravenous and eat large portions of vermicula.